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The Gir Forest, a sere landscape of teak, acacia, and jujube trees in western India, is home to the world’s last remaining wild Asiatic lions. Park rangers track the 650 cats’ every move to protect them, and scientists have been following the endangered population’s triumphs and tragedies since the mid-1990s in one of the longest-running carnivore studies in Asia.
In December 2018, rangers spotted a Gir lioness with an odd litter of cubs: One of them was a young leopard. “I got the information, went there, and just couldn’t believe my eyes,” says Stotra Chakrabarti, a behavioral ecologist at Macalester College in Minnesota, who was then a graduate student doing field work in the area.
Lions and leopards compete for the same resources, so the usual reaction of an adult of either species encountering a youngster of the other would be to kill it on the spot. But for the next 45 days, the rangers and researchers watched as the leopard cub nursed from the lioness, noshed on kills she made, and romped with his adoptive lion siblings.
This offbeat family idyll was short-lived: In February 2019, the leopard cub turned up dead near a watering hole. His small body looked perfect from the outside, unscathed, but an autopsy revealed a femoral hernia—a birth defect that may have caused his biological mother to abandon him and eventually led to his death.
Such relationships seem like an evolutionary paradox at first glance.
Years later, Chakrabarti remains “completely stumped” by the unusual adoption. “I will probably never see this again,” he marvels.
The word adoption “is very anthropomorphically charged,” Chakrabarti acknowledges. Adoption is known widely in human societies but is more likely to be thought of as a behavior that sets us apart from other animal species rather than a practice that unites us. Yet relationships in which an adult animal takes on a parental role for a young one that is not her or his own offspring, while rare, are widely documented across the animal kingdom. Sea otters adopt, as do elephant seals, gulls, dolphins, elephants, cheetahs, penguins, storks, African wild dogs, and a whole troop of primate species.
Such relationships seem like an evolutionary paradox at first glance. Feeding and protecting a juvenile is costly. Evolution is supposed to be about the promulgation of one’s genes; why invest time and energy in the survival of another’s, potentially at the expense of your own? Yet animal adoption may also illuminate the underpinnings of a sensibility common to humans and other animals.
Most animal adoptions occur within species, often within extended family groups. Orphaned chimpanzees may be cared for by older siblings, and among red howler monkeys in Venezuela grandmothers sometimes step in to care for a daughter’s infant. These observations are easily explained by the evolutionary principle of kin selection, which holds that seemingly altruistic behavior is evolutionarily advantageous when it benefits an individual’s relatives, who after all share many of the individual’s genes.
But sometimes the connections underlying adoptive relationships are more nebulous. When a young mountain gorilla loses their mother, the rest of the troop comes together to support them. This includes older siblings, same-age peers, and especially the troop’s dominant male, who will often share his nest to help the youngster stay warm at night.
“Care for the orphan does particularly fall on the dominant male,” says Robin Morrison, an evolutionary biologist at the University of Zurich in Switzerland who also works with the Dian Fossey Gorilla Fund’s Karisoke Research Center in Rwanda. Even if the male is not related to the adopted juvenile, he and his genes may benefit: Female mountain gorillas prefer to mate with males who are good with babies.
Animals sometimes adopt unrelated young even from outside their community. Researchers at the Luo Scientific Reserve in Wamba, Democratic Republic of the Congo witnessed two cases in which female bonobos adopted infants from neighboring social groups. Similar behavior has also been seen in Angola black-and-white colobus monkeys and Taihangshan macaques.
Those adoptions are hard to square with classic evolutionary theory. Harder still are cross-species adoptions. In addition to the lioness who adopted the leopard cub, scientifically documented examples include a troop of capuchin monkeys taking in a baby marmoset; a bottlenose dolphin caring for a melon-headed whale calf alongside her own baby; and an Icelandic killer whale seen traveling alongside a long-finned pilot whale calf. And amateur observations of cross-species adoptions—cats adopting puppies, dogs adopting fawns, and so on—abound in the form of cute animal videos on the Internet, suggesting that the propensity for these relationships may be much more widespread than had previously been acknowledged.
Among Lake Erie’s ring-billed gulls, who breed in large, chaotic colonies, about 8 percent of chicks leave their natal nest each year, sneak into a nearby one, and are accepted by the neighboring parents. This so-called reproductive error, a kind of glitch whereby animals become confused or fail to discriminate between other offspring and their own, is frequently invoked to explain animal adoption. In the big evolutionary scheme of things, it seems, it’s better for gull parents to occasionally raise a chick that’s not their own than to be hypervigilant about interlopers, which could sometimes result in pushing their own young out of the nest.
A similar hullaballoo characterizes the elephant seal birthing colony at Año Nuevo, California, where one-quarter to one-half of pups each year become separated from their mothers due to storms, high tides, or the disruptive movements of galumphing males. About one-quarter of these “orphans” are subsequently adopted or frequently cared for by other females. Most often, the adoptive mothers have lost or become separated from their own pup, and many of them are young and inexperienced. (The Gir lioness, too, was relatively new to motherhood, raising only her second litter of cubs, when she adopted the leopard.)
Lack of experience may contribute to the failure to discriminate between one’s own and other young, but researchers have suggested another benefit to adoption for these mothers: It’s good practice, yielding improved parenting skills that help the animal’s own young later on.
Surging maternal hormones may contribute to a new mother’s willingness to take responsibility for an unrelated mouth to feed, either in addition to her own young or as a substitute if she has recently lost a baby. Most of the time, an animal’s intense maternal instincts will be directed at her own offspring, says James Serpell, professor emeritus at the University of Pennsylvania School of Veterinary Medicine. But every so often, she’ll simply “glom on to whatever comes in handy.” This may sometimes lead to Raising Arizona-style capers documented in the scientific literature in which animals adopt young who don’t actually need adopting—a Tibetan macaque mother, for example, who had recently given birth, kidnapped another infant and ended up raising both babies as her own.
Maybe our tendency is to look at evolutionary forces too narrowly.
In the southwestern United States, biologists working to restore endangered Mexican wolves have put these widespread proclivities to use in a fostering program that places captive-born wolf pups into wild dens with litters of the same age.
“We rub them all together in this little puppy pile” so that captive-born pups acquire their newfound siblings’ scent, explains Allison Greenleaf, a senior U.S. Fish and Wildlife Service biologist on the Mexican wolf recovery program. Then the new, expanded litter is put back in the wild den. Mother Nature—or mother wolf, with assistance from the rest of the pack, who cooperate to raise a single breeding pair’s litter—does the rest.
The fostering program, the researchers hope, will enrich the genetic diversity of the wild Mexican wolf population, which is descended from just seven captive-raised individuals. It’s also thought that wild-raised pups will be more wary of humans and thus less likely to become nuisance animals than captive-raised wolves released into the wild as adults, Greenleaf says.
Since 2016, 126 captive-born Mexican wolves have been fostered into 48 dens, and at least 20 have survived to reach breeding age at 2 years old—a survival rate comparable to that of wild-born pups raised by their biological parents. Fostered pups have produced 30 litters of their own, and some have even become foster parents themselves.
Adoption may be more complex, but also simpler, than this search for evolutionary explanations makes it out to be. People reflexively look to immediate biological advantages as a rationale for animal behavior, says Judith Benz-Schwarzburg, an ethicist at the University of Veterinary Medicine in Vienna, Austria. But adoption shows that other animal species, especially those who care for their young, are “driven by lots of reasons”—just like humans are.
Chakrabarti, the biologist who studied the leopard cub-adopting lioness, would agree. Animals are not “automatons,” he says. “Each of them are rational, sentient beings,” with individual agency, preferences, and personality. We may never know why some feel moved to adopt.
Or maybe our tendency is to look at evolutionary forces too narrowly, on too fine a timescale. Humans and other animals with extended parental care have “had hundreds of thousands of years of evolutionary pressure to care for infants and look after them,” Morrison says. “Having strong caring instincts has been really valuable across our evolutionary history.”
Those evolutionary forces will be especially strong in the case of species, like our own, that exhibit cooperative parental care. Perhaps that helps explain why we’re the absolute champions at cross-species adoption. Although dogs and cats might have started as working additions to households, many have become more akin to “fur babies,” as the contemporary term goes; in many hunter-gatherer societies it’s common for people to bring young animals into the village and raise them as pets. This behavior may be “a natural consequence of a human propensity to care for other individuals,” as Serpell puts it.
In fact, one theory of dog domestication holds that the process began with Pleistocene hunter-gatherers bringing young wolf pups, perhaps orphans, back to camp to be suckled by human women and raised as part of the community.
An early domestication of dogs might have even facilitated other cross-species adoptive relationships. “I strongly suspect that the capacity of a nursing mother dog to accept puppies that aren’t her own may have helped in the domestication of the cat,” Serpell says. “Adult dogs are not very accepting of cats, and it would have been difficult to domesticate cats in a community that already had dogs. But if you had a nursing mother dog, you could certainly foster a kitten onto her and she would adopt it.”
Several thousand years later, these phenomena remain embedded in our daily lives. Consider the three cats who live in my house: two full sisters and a half-sister who is a few months older, we’ve been told, with a foundling backstory involving a plastic tote in the woods. As in orphan chimpanzee families, the older girl has become a mother figure for the younger two, grooming them, putting them in their place with a swat when necessary, patrolling the ramparts of the windowsills for threats to her territory.
Sometimes I watch her, stretched out on her side in the pose of a nursing mother cat, the other two lying crosswise on top of her even though all three are now fully grown. In her posture, at once patient and resigned, I can see the echoes of a lioness in a clearing in the Gir Forest. I can see a half-wild wolf suckling a half-wild kitten, and I can see a fully wild wolf unfazed by the sudden growth in the size of her litter.
Then again, maybe my own devotion to the older sister, my desire to see the caretaker well cared for, also has something in common with that lioness. Maybe animal adoptive parents are not just a curiosity but a model for human behavior: we affectionate silverbacks, we Gir lionesses, weaving relationships both genetic and not, united by a drive to care. 
Lead image: vvvita / Shutterstock
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